What is a derecho?

A massive line of fierce storms forecast to wallop the Midwest and Mid-Atlantic on Wednesday and Thursday has meteorologists warning about the possibility of a derecho.

But what exactly is a derecho, and why is it so dangerous? Click through for more information about this weather event.

A derecho is a widespread, long-lived wind storm associated with a band or shelf of rapidly moving thunderstorms. They arise when huge downbursts of cold air hit the ground, spawning winds that spread out in straight lines from the point of impact.

"Derecho" is a Spanish word, meaning "right" or "straight." The term was coined in 1888 to describe a severe straight-wind storm, in contrast to a tornado.

To be classified as a derecho, the swath of wind damage must extend more than 240 miles, and winds must be greater than 57 mph. Derecho winds can range well beyond 100 mph.

The prime season for derechos runs from May to August. Early-season hot spots are Oklahoma, Kansas, Missouri and Arkansas. Later in the season, the potential shifts to southern Minnesota, the Great Lakes and the Ohio Valley region.

Roughly every four years, a derecho breaks out of the Midwest, crosses the Appalachian Mountains and heads for the Atlantic without dissipating.

That's what happened last year, when a powerful storm system blasted from Indiana to Maryland. Considered the largest of its kind, it killed more than a dozen people, leaving millions in the dark and shutting down Netflix as well as other online services that relied on Amazon's Cloud servers.

The derecho that struck southeastern Kansas, southern Missouri and southwestern Illinois in May 2009 was the first to be classified a "Super Derecho." Thirty-nine tornadoes were reported, along with baseball-sized hail and wind gusts of 106 mph reported in Carbondale, Ill.

Another derecho is thought to have occurred in North Carolina on April 15, 1999, producing a killer tornado and a 165 mph wind gust, although this may have been associated with a tornado.

Derecho warnings usually come hours in advance, and meteorologists urge those in a derecho's path to take precautions similar to an approaching tornado or other severe storm -- secure loose items outside, bring in furniture and other equipment, seek shelter in a sturdy structure and stay away from windows.

Click here for more information on derechos from the National Weather Service.

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